As is known, electroluminescent-unit display devices, such as for example those using organic light-emitting diodes (OLEDs), comprise, deposited on a semiconductor substrate:                a luminous emission region formed by a light-emitting structure that comprises at least one organic film and which is interposed between two, one internal and one external, electrodes at least one of which is transparent or semitransparent to the emitted light, the external electrode being thin or relatively thick depending, respectively, on whether emission is to be via the external face of this region or through the substrate; and        an electrical contact region that is generally arranged adjacent this emission region and that comprises at least one region of contact with the internal electrode and one region of contact with the external electrode, each region possibly being physically formed from a plurality of connections so as to minimize the access resistances.        
The encapsulation of OLED units is a critical subject and has been the cause of much research. These units are usually protected from moisture and from oxygen in the ambient air in two different ways:                either by a protective cap, for example made of glass, molded so as to have a peripheral edge that protrudes toward the interior and bonded only by this edge to the periphery of the emission region of the device, so that the pressure involved in assembly does not alter the OLED stack;        or by one or more thin encapsulation layer(s) deposited on the emission region and typically comprising organic and inorganic materials in alternation so as meet the required permeability specifications.        
One major drawback of the aforementioned first method of encapsulation resides, on the one hand, in the need to shape the protruding edge of this cap during the molding and, on the other hand, in the reduced area of the bonding interface obtained, which is not always able to guarantee an effective, long-lasting encapsulation.
Concerning the aforementioned second method of encapsulation, it is known, for OLED units emitting via the external (i.e. top) face, to deposit on the external electrode a thin capping layer that is transparent to visible light, and which is generally made of a dielectric having a refractive index higher than 1.8 so as to extract photons from the OLED unit. This capping layer must furthermore be deposited using a “soft” method (i.e. using a reduced temperature and a very low-energy bombardment in the presence of few oxidizing species and little water) such as vacuum evaporation, so as not to alter the electro-optical properties of the underlying organic semiconductors.
The capping layers thus deposited have the drawback of being insufficiently dense and of not being “conformal” enough (i.e. of not satisfactorily following the micro- or nano-reliefs of the surfaces that they cover), which reduces the effectiveness of the encapsulation.
Document US-A-2002/0003403 provides an OLED device having two encapsulation layers superposed on the emission region and respectively made of a layer of dielectric oxide, for example deposited using ALD (atomic layer deposition), and a polymer layer, for example made of parylene. Mention may also be made of documents US-A-2007/0099356 and US-A-2007/0275181 that teach the use of the ALD technique for depositing an encapsulation layer on electroluminescent units.
These ALD deposited encapsulation layers have the advantage of being “conformal” whilst having a high density, which affords them relatively satisfactory barrier properties with respect to ambient moisture and oxygen. Nevertheless, one drawback of these ALD deposited layers lies in the use of ALD deposition precursors that contain oxidizers and water, which are liable to alter the characteristics of the underlying OLED unit. The precursors are all the more reactive since the ALD deposition is carried out at low temperature.